1 Field of the Invention
The present invention relates to a flame-retardant adhesive film that can be used favorably in the production of a flat cable, and to a flat cable produced from this film.
2 Description of the Related Art
There are known flat cables with a construction in which a plurality of conductors in a line shape covered with two adhesive films in which an adhesive layer is formed on a substrate film. These flat cables are widely used for the wiring of the AV and computer devices which have recently come to be mounted at such high densities.
From the standpoints of both their electrical applications and fire prevention, the material that makes up a flat cable other than the conductors (that is, the adhesive film composed of a substrate film and an adhesive layer) needs to exhibit good electrical insulation and good flame retardance.
Accordingly, halogen-based flame retardants (such as decabromodiphenyl ether or hexabromodiphenyl ether), as flame retardance auxiliaries inorganic fillers such as silica, clay or the like have been added to the adhesive layer of the adhesive films used in flat cables. However, non-halogen-based flame retardants have recently come to be used in place of halogen-based flame retardants as the flame retardants added to the adhesive layer in an effort to protect our global environment. For instance, Japanese Patent application Laid-Open No.06-338225 discloses that a phosphoric ester or another such a phosphorus-based flame retardant is added as a non-halogen-based flame retardant to the polyester flame-retardant layer of an adhesive tape used for a flat cable. Also, Japanese Patent application Laid-Open No.09-221642 discloses that a thermoplastic polyester resin substrate that makes up an adhesive film used for a flat cable, or an adhesive layer formed over this substrate, is rendered flame-retardant by a phosphorus-based flame retardant.
When flat cables are used for the wiring of various products related to human safety (such as airbags in automobiles or medical equipments), extremely high electrical insulation is required in order to ensure a consistently high level of safety.
A problem with the adhesive tapes or adhesive films disclosed in the above publications, however, is that sufficiently high insulation resistance of the adhesive tape or film cannot be achieved because the phosphorus-based flame retardant itself contained in the adhesive layer has hygroscopic or ionic properties.
One possible way to deal with this problem is to add a nitrogen-containing organic flame retardant that exhibits relatively high insulation performance (such as melamine) to the adhesive layer of a flame-retardant adhesive film, but because the flame retardance of a nitrogen-containing organic flame retardant is insufficient, the resulting flame retardance is not on a par with that of a flame-retardant adhesive film containing a halogen-based flame retardant. Moreover, the necessary adhesive strength cannot be ensured if a large amount of nitrogen-containing organic flame retardant is added to the adhesive layer in order to increase flame retardance. Consequently, nitrogen-containing organic flame retardant are not as yet being used with flame-retardant adhesive films for use in flat cables.